DC-DC converters are a class of electronic circuit whose input is one DC voltage level--not necessarily constant--and whose output is another DC level. A load is connected to the output, and the converter, in addition to converting the input voltage level to the output level, usually regulates the output level. This means it maintains the output voltage approximately constant against changes in the input level or changes of load value. (The term "regulator" is often used interchangeably with converter.)
Early designs for such converters commonly consisted of a variable resistance inserted between the source supplying the input level and the load. A control circuit monitored the voltage across the load and dynamically adjusted the variable resistance to maintain the load voltage at the desired level.
More recently, switching DC-DC converters have greatly increased in popularity. Typically, a switching converter uses an electronic switch to chop the source voltage. The resultant pulsed waveform is filtered to reduce its AC component, and thus its average (DC) value is supplied to the load. The average value is controlled through varying the pulsed waveform; for example, changing its duty cycle, or omitting some of the pulses. Switching converters/regulators are usually more efficient than the variable resistance type, and this is a major reason for their popularity. In addition, with proper converter topology, output voltages higher than the input, or of opposite polarity, may be generated. Application Note AN920 from Motorola, Inc. is a good source both for understanding switching regulators and for a background to principles of the present invention. This Note, "Theory and applications of the MC34063 and .mu.A78S40 switching regulator control circuits", is hereby incorporated by reference in this disclosure.
FIG. 1 is similar to FIG. 1b of the Motorola reference, and shows a simplified switching DC-DC converter. A DC source Vin is applied, via a switching transistor Q1, to a filter network consisting of elements D1, L1, and C1. A load R1 dissipates the filtered power at a voltage level Vout. When Vout (scaled by R2 and R3) is less than a reference voltage REF, the comparator enables the gated latch. The latch accepts pulses from the free-running oscillator and delivers a pulsed drive to the base of Q1. When V1 is thus switched on and off, it supplies a pulse train of height Vin to the filter network. However, when the filtered value Vout of this train rises above its nominal value, the comparator disables the latch, thus turning off Q1. As R1 draws energy from the filter network, Vout decays below its nominal value and the comparator re-enables the latch. This "bang-bang" control circuit holds Vout close to its nominal value even if Vin and/or R1 change.
The converter design scheme shown in the Motorola reference is effective, yet there are some operating states of the circuit which cause difficulty.
One of these is circuit startup: when power is first applied, the circuit must bring up the output voltage from zero to Vout. This means pumping charge into C1 until its terminal voltage reaches Vout. Since the voltage across C1 is initially zero, L1 is subject to the full value of voltage Vin, resulting in current pulses through Q1 and the filter elements which can reach damaging levels much higher than those experienced in steady-state operation.
Another difficulty can occur when the load R1 suddenly changes value from very large to very small, or vice versa. This is a commonplace occurrence in thermal inkjet printers, the design environment in which the present invention was conceived. In these printers, large current pulses of short duration are applied to ink-propelling resistors. Although the printer controller "knows" in advance when these loads are to occur, this information is not used to prepare the converter to allow better handling of the resulting transients.
Yet another problem results when design requirements change or when component values must be modified. Unless corresponding changes are made in the duty cycle and/or the frequency of the oscillator, the circuit in its new state may operate at less than best efficiency.
Operational problems such as these reveal the need for a converter circuit whose characteristics can be modified dynamically to match more closely actual or anticipated operating state changes, whether short-term, such as start-up, or long term, such as different values of filter components.